1. Field of the Invention
The present invention relates to a brushless motor drive circuit, and particularly to a means for reducing the switching pulse during phase switching.
2. Description of the Prior Art
The good control characteristics of recent compact DC motors have been confirmed in the audio, information processing, and various industrial fields, leading to the rapid development of applications for such motors. Brushless motors in particular have a long service life because they have no contact parts such as brushes and commutators. Industrial applications for brushless motors, for which reliability is a primary consideration, have therefore expanded rapidly.
An example of one such application is the axial-flow fan, the drive method of which has changed in recent years from AC to DC motors. DC axial-flow fans using a brushless motor are now common.
DC axial-flow fans have two design constraints: the drive circuit must be housed in limited space, and the fan must be marketable at a low cost. To meet these constraints, a single magnetic detection element is used for drive, and simple two-phase, half-wave wiring is used. To further simplify the circuitry and improve efficiency, a method whereby the output circuit switches, and the generated switching pulse is suppressed by a capacitor, Zener diode, or other device, is commonly used.
FIG. 7 is a schematic diagram of a brushless motor drive circuit using a capacitor according to the prior art; FIG. 9 is a wave diagram of the output wave from the drive circuit in FIG. 7 and the output circuit in FIG. 8, which is described below.
As shown in FIG. 7, the base of transistor 112 is connected to the output terminal of Hall IC 111; the collector of the transistor 112 is connected to one end of resistance 113, the other end of which is connected to Vcc, and to the anode of diode 114, which is connected in series with diode 115; and the emitter is connected to one end of resistance 116, the other end of which is connected to the GND, and to the base of output transistor 117,.the emitter of which is also connected to the GND.
The collector of the output transistor 117 is connected to one side of the capacitor 118, the other end of which is connected to the GND, and to Vcc through the motor drive coil 119.
The emitter of the output transistor 121 is connected to the GND; the base to the cathode side of diode 115, and to one side of resistance 120, the other side of which is connected to the GND; and the collector is connected to Vcc through the motor drive coil 123, and to one side of capacitor 122, the other side of which is connected to the GND.
The output circuit 124 is the area enclosed in the dotted line.
With the drive circuit shown in FIG. 7, however, the output current wave shown in FIG. 9A will become an output voltage wave with a switching pulse (dotted line indicates a virtual induced electromotive force) as shown in FIG. 9B generated during phase switching. This switching pulse generated during phase witching can be reduced by ptimally selecting the capacitance of capacitors 118 and 122, resulting in the switching pulse (dotted line indicates a virtual induced electromotive force) shown in FIG. 9C, but this is not truly satisfactory. More specifically, no benefit is obtained if the capacitance of capacitors 118 and 122 is too low; if the capacitance is increased too much, a time delay occurs as shown in FIG. 9C, leading to problems with switching timing, or current backflow to the power supply caused by an increase in resonant energy with the motor drive coil. A compromise capacitance of several microfarads (1-2 .mu.F) is therefore generally used.
Because it is necessary to place the circuit components in a confined space in DC axial-flow fan motors, it is necessary to reduce the number of external components and increase the use of ICs. There is also a method using Zener diodes in place of capacitors, and thus does not use the two several-microfarad capacitors 118 and 122 shown in FIG. 7.
FIG. 8 is a schematic diagram of another brushless motor drive circuit, an output circuit in which one phase part of the output circuit 124 shown in FIG. 7 is changed. Specifically, the emitter of output transistor 134 is connected to the GND; the collector is connected to Vcc through the motor drive coil 135, and to the cathode side of the Zener diode group 133; and the base is connected through resistor 131 to the GND, and through resistor 132 to the anode side of Zener diode group 133.
While the drive circuit can be easily integrated using this design, this design conversely increases the switching noise (dotted line indicates a virtual induced electromotive force) generated during phase switching as shown in FIG. 9D.